Fuse circuits are frequently used in various semiconductor applications. For example, as the storage density of semiconductor devices such as memories, programmable logic devices, and the like continue to increase, the incidence of defective cells within any one device is likely to increase as well. Unless the defect can be corrected in some way, the particular defective semiconductor device becomes useless, thus lowering production yields. A common approach to correcting such defects is to provide a multitude of redundant cells within the device. When a defective cell is detected, one of the redundant cells can be used in its place. The circuitry that supports the redundant cells usually include fuse circuits containing fusible links which are selectively "blown" in order to activate one or more of the redundant cells.
Fuse circuits are also used to facilitate programming of programmable logic devices (PLD). Typically, logic cells in a PLD are fabricated to have a default logic level, whether it be a logic HI or a logic LO. This is accomplished by the presence of fusible links which tie the cells' outputs either to V.sub.cc or to ground. When a fusible link in a cell is blown, the cell's output reverses.
An important consideration with such fusible links is that they consume power. An especially problematic situation exist when a fuse is partially or not completely blown. The result is a current path across the fuse, which is undesirable for two reasons: First, the partial fuse is likely to create an indeterminate connection state. Depending on the resistance of the partially blown fuse, the surrounding circuitry may "see" an open or a short. Consequently, the device is not likely to behave as expected. Second, even if the circuitry sees an open circuit across the partially blown fuse, there will be a current leakage across the fuse. The amount of current can be 1 .mu.A or more which can add up to a significant current draw for a typical application employing redundant circuits.
What is needed is a fuse circuit which can reliably provide a completely "blown" state. It is desirable that the fuse circuit behave as if it were completely blown even though the fusible link is in fact partially blown.